HECT-type ligases facilitate autoubiquitination and degradation of other ubiquitin ligases to activate plant immunity
The ubiquitin-proteasome system (UPS) serves as the primary proteolytic machinery in eukaryotes, governing intracellular protein turnover to maintain proteome homeostasis. In plants, the HECT-type UPL3/4 ubiquitin ligases play vital roles in developmental and immune signaling. After ubiquitination by pathway-specific E3 ligases, substrates are physically relayed to proteasome-associated UPL3/4 ligases for further modification, which is necessary for their proteasome-mediated degradation. In this study, we investigated if the cellular influence of UPL3/4 extends beyond their direct role in substrate degradation. We discovered that UPL3/4 govern the ubiquitination not only of a broad array of immune-related substrates, but also of many UPS components, including E3 ligases. UPL3 physically interacts with PUB22, a pathway-specific U-box E3 ligase that negatively regulates immunity. PUB22 is controlled by a phospho-switch that converts it from an instable autoubiquitinated state to a stable phosphorylated E3 ligase that marks substrates for degradation. Remarkably, UPL3 only interacted with unphosphorylated PUB22 and facilitated its autoubiquitination-mediated degradation, thereby promoting the accumulation of PUB22 substrates. Moreover, the compromised immune phenotypes of upl3 upl4 mutant plants were largely dependent on PUB22 and its close paralogues. Thus, UPL3/4 control the stability of immune-related substrates not only through direct ubiquitination, but also indirectly by promoting autoubiquitination of PUB22 ligase and its paralogues. Controlling the stability of autoubiquitinating E3 ligases may be a universal mechanism whereby HECT-type ligases and the proteasomes they associated with, orchestrate cellular proteostasis in eukaryotes. Significance StatementThe ubiquitin-proteasome system (UPS) governs intracellular protein turnover to maintain proteome homeostasis in eukaryotes. Proteasome-associate HECT-type ubiquitin ligases play an important role in processing and degrading substrates delivered to the proteasome by pathway-specific E3 ligases. Here, we discover that in plants, HECT-type ligases not only promote the degradation of substrates, they also modify the E3 ligases that target these substrates to the proteasome. Specifically, HECT-type ligases facilitated or expanded the autoubiquitination of immune-suppressive E3 ligases, resulting in their proteasome-mediated degradation and onset of immunity. Our discoveries suggest that during plant immunity, HECT-type ligases and the proteasomes they associate with, control cellular proteostasis by governing the stabilities of both E3 ligases and their substrates.